All high power solid-state lasers experience some distortion of the laser beam due to temperature and stress-induced changes in the refractive index of the lasing material. In the case of cylindrical geometry lasers, the non-uniform temperature distribution is radially parabolic, resulting in a thermal lensing of the beam. Although a fixed focal length spherical optic can correct this aberration at one operating point, the degree of thermal lensing generally varies in proportion to the level of optical pumping, or heat dissipated. Therefore, a variable powered optic is sometimes necessary to avoid mode instabilities which can result in poor mode/gain overlap and intra-resonator focusing of the beam which can damage optics.
In long, thin solid-state gain elements, like rods, temperature dependent refractive index is the primary aberrating mechanism. However, in very thin gain elements, like disks, thermal bending of the lasing medium due to stress buildup is predominant—yet is still largely a parabolic wave-front distortion in nature. This effect is compounded when multiple rods or disks are used serially in the resonator. A variable power optical element is therefore used for high power solid-state lasers and amplifiers to correct thermal focus aberrations. Commercial-off-the-shelf (COTS) devices are not available to provide this correction.
Traditional adaptive optics may use a large area to achieve amplitude correction. This may involve the incorporation of beam expanders and telescopes into the beam train, which can greatly complicate problems and increase costs. Conventional bimorph mirrors may not operate with high wave-front quality at the flux levels required. Other approaches to wave-front correction and high power beams may employ active cooling or other heat removal methods, which can also add complexity and increase costs.
As a result, there is a need for focus correction of solid state lasers under varying thermal and power operating conditions.